Global demand for liquid transportation fuel is projected to strain the ability to meet certain environmentally driven goals, for example, the conservation of oil reserves and limitation of greenhouse gas emissions. Such demand has driven the development of technology which allows utilization of renewable resources to mitigate the depletion of oil reserves and to minimize greenhouse gas emissions.
Butanol is an important industrial chemical, useful as a fuel additive, as a feedstock chemical in the plastics industry, and as a food grade extractant in the food and flavor industry. Each year 10 to 12 billion pounds of butanol are produced by petrochemical means and the need for this commodity chemical will likely increase in the future.
Methods for the chemical synthesis of isobutanol are known, such as oxo synthesis, catalytic hydrogenation of carbon monoxide (Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, 2003, Wiley-VCH Verlag GmbH and Co., Weinheim, Germany, Vol. 5, pp. 716-719) and Guerbet condensation of methanol with n-propanol (Carlini et al., J. Molec. Catal. A: Chem. 220:215-220, 2004). These processes use starting materials derived from petrochemicals, are generally expensive, and are not environmentally friendly. The production of isobutanol from plant-derived raw materials would minimize greenhouse gas emissions and would represent an advance in the art.
2-Butanone, also referred to as methyl ethyl ketone (MEK), is a widely used solvent and is the most important commercially produced ketone, after acetone. It is used as a solvent for paints, resins, and adhesives, as well as a selective extractant, activator of oxidative reactions, and it can be chemically converted to 2-butanol by reacting with hydrogen in the presence of a catalyst (Nystrom, R. F. and Brown, W. G. (J. Am. Chem. Soc. (1947) 69:1198). 2,3-butanediol can be used in the chemical synthesis of butene and butadiene, important industrial chemicals currently obtained from cracked petroleum, and esters of 2,3-butanediol may be used as plasticizers (Voloch et al., “Fermentation Derived 2,3-Butanediol,” in Comprehensive Biotechnology, Pergamon Press Ltd., England Vol. 2, Section 3:933-947 (1986)).
Microorganisms can be engineered for the expression of biosynthetic pathways that initiate with cellular pyruvate to produce, for example, 2,3-butanediol, 2-butanone, 2-butanol and isobutanol. U.S. Pat. No. 7,851,188 discloses the engineering of recombinant microorganisms for production of isobutanol. U.S. Patent Application Publication Nos. US 20070259410 A1 and US 20070292927 A1 disclose the engineering of recombinant microorganisms for production of 2-butanone or 2-butanol. Multiple pathways are disclosed for biosynthesis of isobutanol and 2-butanol, all of which initiate with cellular pyruvate. Butanediol is an intermediate in the 2-butanol pathway disclosed in U.S. Patent Application Publication No. US 20070292927 A1.
The disruption of the enzyme pyruvate decarboxylase (PDC) in recombinant host cells engineered to express a pyruvate-utilizing biosynthetic pathway has been used to increase the availability of pyruvate for product formation via the biosynthetic pathway. For example, U.S. Application Publication No. US 20070031950 A1 discloses a yeast strain with a disruption of one or more pyruvate decarboxylase genes (a PDC knock-out or PDC-KO) and expression of a D-lactate dehydrogenase gene, which is used for production of D-lactic acid. U.S. Application Publication No. US 20050059136 A1 discloses glucose tolerant two-carbon source-independent (GCSI) yeast strains with no PDC activity, which may have an exogenous lactate dehydrogenase gene. Nevoigt and Stahl (Yeast 12:1331-1337 (1996)) describe the impact of reduced PDC and increased NAD-dependent glycerol-3-phosphate dehydrogenase in Saccharomyces cerevisiae on glycerol yield. U.S. Application Publication No. 20090305363 A1 discloses increased conversion of pyruvate to acetolactate by engineering yeast for expression of a cytosol-localized acetolactate synthase and substantial elimination of PDC activity.
While PDC-KO recombinant host cells can be used to produce the products of pyruvate-utilizing biosynthetic pathways, PDC-KO recombinant host cells require exogenous carbon substrate supplementation (e.g., ethanol or acetate) for their growth (Flikweert et al. 1999. FEMS Microbiol. Lett. 174(1):73-79 “Growth requirements of pyruvate-decarboxylase-negative Saccharomyces cerevisiae”). A similar auxotrophy is observed in Escherichia coli strains carrying a mutation of one or more genes encoding pyruvate dehydrogenase (Langley and Guest, 1977, J. Gen. Microbiol. 99:263-276).
In commercial applications, addition of exogenous carbon substrate in addition to the substrate converted to a desired product can lead to increased costs. There remains a need in the art for recombinant host cells with reduced or eliminated need for exogenous carbon substrate supplementation.